Non-point source pollution is pollution that enters water from many different sites, rather than from just one site. Examples of non-point source pollution are contaminated rain falling from the sky, polluted melting snow, runoff (water flow on land) of polluted water, and impure water draining down into the groundwater from many different sites on the surface. In contrast, an example of point source pollution is a polluted river flowing into a lake.

Because non-point source pollutants enter a water body such as a stream, river, or lake at different locations, the control and prevention of non-point pollution can be much more difficult than when the contaminants are entering at a single site.

As the water runs over the land or through the ground on its way to the body of water, it can pick up a variety of pollutants. These chemicals and undesirable microscopic organisms in the water pollute the water into which they flow.

The flow of the polluted water can also harm plants and carry soil into the water, which can change the shape and flow of the current (steady flow of water in a prevailing direction) in a water body.

According to the U.S. Environmental Protection Agency, about 40% of all U.S. freshwater sources that have been surveyed (mapped) have been damaged by non-point source pollution. The damage in many areas is enough that the water is unhealthy for swimming and fish caught from the water should not be eaten.

Types of non-source pollutants

Sediment Sediment is small particles of soil, rock, or dirt carried and deposited by water. This material can enter water from many sources, such as fields, construction sites, mining or logging operations that scour off surface vegetation, and erosion (wearing away) of riverbanks or other land.

Nutrients
A nutrient is a food source and all organisms need these to survive. Non-point source pollution carries a greater flow of nutrients into a water body from croplands, nurseries, orchards, livestock and poultry farms, lawns, and landfills that can disrupt the balance of life in the water. This disruption can change the chemistry of the water so that it is no longer able to sustain fish and plant life.

Heavy metals
Heavy metals, such as lead or mercury, are poisonous if present in too high a concentration in the body. Fluids that leak out of vehicles and runoff from mine sites, roads, and parking lots can all contain heavy metals.

Toxic chemicalsToxic (poisonous) chemicals can enter water from the runoff from farmland, nurseries, orchards, construction sites, and lawns and landfills. They can kill or harm organisms in the water, such as fish, and sicken animals and people who eat these organisms. In 2002 scientists found unhealthy levels of mercury in salmon taken from waters off the Pacific Northwest and urged the public to limit consumption of salmon to one serving per week.

Pathogens
Pathogens are microorganisms that can cause disease. Examples of pathogens include certain types of bacteria, viruses, and protozoa. Water that is contaminated with human or animal feces (such as sewage, waste from farms, and fluid leaking from landfills) often contains harmful microorganisms. If the microorganisms are not removed from the water then people who drink the water can become ill.

Origins of non-source pollution

Urban areas

Urban areas

Cities and towns contain a lot more people per area than rural areas and many people in one place mean that there are more potential sources of pollution. In urban areas one of these sources is the home.

When a lawn is sprayed with pesticide (chemicals to harm pests) to kill weeds, rain or other water can cause of the poison to flow into the sewer pipes, which drain excess water off the streets.

In many cities the sewer water runs directly into a nearby water body. Fertilizer that is applied to the lawn to help grass grow can contain nutrients for organisms in the water (such as nitrogen and phosphorus). The pollution of water by fertilizer is also a big problem in agricultural areas.

The waste left behind on the lawn by the family dog can be washed away. This fecal material can contain pathogens. Water that is flushed down toilets or drains from sinks can also contribute to pollution.

Even though many communities will treat this wastewater, some contaminants can make it through the treatment process. For example, studies that have been done in the 1990s have found oil, grease, some harmful metals, and even antibiotics (chemicals that kill bacteria) in the water that leaves treatment plants.

Another major source of non-point source pollution in more northern urban areas is the salt that is spread to keep roads free of ice during winter.

In the city of Toronto, Canada, the runoff of salt into Lake Ontario was so great one winter in the 1990s that scientists found that portions of the lake contained plants that are normally found along ocean coastlines. Also, oil and gasoline can seep from cars and trucks. These pollutants are washed away and will end up in a nearby water body.

Rural and industrial areas

The nitrogen and phosphorus components of many fertilizers, which are food sources for the crops, can also be a rich source of food to microorganisms such as algae. The rapid growth and huge increase in the numbers of the microorganisms can use up much of the oxygen in the water.

Creatures such as fish that depend on oxygen for their survival will die. As well, the lack of oxygen disrupts the normal chemical processes that keep the water healthy. This process is called eutrophication.

In some areas of North America farms contain tens of thousands of poultry or pigs. The waste material and huge amounts of water used in the farm operation are typically stored in large lagoons (shallow bodies of water that are separated from the sea by a reef or narrow island).

There have been cases where a lagoon wall has ruptured, allowing the contaminated water to pour into a nearby stream or river, or to seep down into the groundwater. Much effort is being directed at trying to find better and safer ways to store and safely dispose of this contaminated water.

Control of non-point source pollution

Although it is impossible to prevent all runoff from entering water, steps can be taken to reduce the pollutants in the runoff. At a national level the U.S. federal government created water pollution control measures in 1972 known as the Clean Water Act.

The regulations of the act help restrict the harmful compounds that enter water, by setting acceptable water quality standards, by making the presence of certain compounds in water illegal and by penalizing polluters. States and communities can also have their own standards and regulations.

In 1990 the federal government also passed legislation known as the Coastal Zone Act Reauthorization Amendments, which was directed at the problem of the non-point source pollution of coastal waters.

The regulations help federal and state officials manage the development and use of land that borders the streams, lakes, and rivers that empty into the sea. The intent is to make the freshwater that enters the sea as free of pollution as possible.

There are actions that everyone can take to reduce non-point source pollution. Alternatives can be found to many toxic cleaners and other household chemicals. Lawn care products need not be toxic to water; environmentally friendly weed killers can be used, as can the manual way of pulling weeds by hand.

When toxic chemicals need to be disposed of they should be taken to a facility, such as a fire station, that can safely deal with the chemicals rather than dumping the liquid down the drain Picking up after the dog is another way to reduce microorganisms from getting into water bodies.

Finally, learning more about water pollution and taking actions both personally and by helping change the behavior of others can help reduce non-point source pollution.